Divergence of insulin-like growth factors I and II in the elasmobranch, Squalus acanthias

Recent studies have shown that vertebrates, including teleostean fishes, amphibians, birds and mammals, contain two distinct insulin-like growth factor (IGF) genes. In contrast agnathans, represented by hagfish, apparently have only one IGF that has features characteristic of both IGF-I and IGF-II. Between these groups the elasmobranchs occupy a critical position in terms of the phylogeny of IGFs. We sought to determine if gene duplication and divergence of IGF-I and IGF-II occurred before or after divergence of elasmobranchs from other vertebrates by cloning IGF-like molecules from Squalus acanthias. Our analysis shows that Squalus liver produces two distinct IGF-like molecules. One has greater sequence identity to, and conserved features characteristic of, known IGF-I molecules, while the other is more IGF-II like. These results suggest that the prototypical IGF molecule duplicated and diverged in an ancestor of the extant gnathostomes.